Typical processes for preparing polybenzimidazoles are described in Journal of Polymer Science, 50, 511 (1961), and in various United States Patents.
U.S. Pat. No. 3,174,947 (No. Re. 26,065) describes a method of preparing high molecular weight aromatic polybenzimidazoles by melt polymerizing an aromatic tetraamine and a diphenyl ester or an anhydride of an aromatic dicarboxylic acid at an elevated temperature and thereafter further polymerizing the product of the melt polymerization in the solid state. According to this process, in order to produce polymers of sufficiently high molecular weight to be suitable for practical use it is necessary to finely pulverize the product of the melt polymerization prior to polymerization in the solid state and to conduct the solid state polymerization at an elevated temperature under a reduced pressure of less than 0.5 mm Hg or at an elevated temperature and in an inert gas stream over a prolonged period of time. Thus, the process requires several complicated operations. In addition, since the reaction is conducted over a long period of time at an elevated temperature, it tends to form insoluble and infusible polymers.
U.S. Pat. No. 3,313,783 describes a process adapted to overcome the above-mentioned deficiencies which process involves the solution polymerization of an inorganic acid salt of an aromatic tetraamine and a dicarboxylic acid or a derivative thereof with heat in polyphosphoric acid. After completion of the reaction the polymer product is separated by pouring the reaction mixture in the form of a polyphosphoric acid solution into a large quantity of water. However, this separation procedure is complicated and it is difficult to recover and reuse the polyphosphoric acid.
Another process for producing polybenzimidazoles is described in U.S. Pat. No. 3,509,108. In this process the monomers are initially reacted in a melt phase polymerization at a temperature below above 200.degree. C. and a pressure above 50 psi. The reaction product is then heated in a solid state polymerization at a temperature above 300.degree. C. to yield the final aromatic polybenzimidazole product. The process requires that the initial reaction be conducted at a pressure above 50 psi (preferably, between 300-600 psi) in order to control the foaming encountered during the polymerization.
U.S. Pat. No. 3,555,389 describes a two stage process for the production of aromatic polybenzimidazoles. The monomers are heated at a temperature above 170.degree. C. in a first stage melt polymerization zone until a foamed prepolymer is formed. The foamed prepolymer is cooled, pulverized, and introduced into a second stage polymerization zone where it is heated in the presence of phenol to yield a polybenzimidazole polymer product. As with the process of U.S. Pat. No. Re. 26,065, this process involves multiple operations and tends to form insoluble polymers.
U.S. Pat. No. 3,433,772 describes a two stage polymerization process for the production of aromatic polybenzimidazoles which utilize an organic additive, such as an alkane having 11-18 carbon atoms or a polycarbocyclic hydrocarbon, in order to control foaming during the first stage.
Other United States Patents relating to one stage and two stage production of polybenzimidazoles include U.S. Pat. Nos. 3,408,336; 3,549,603; 3,708,439; 4,154,919; and 4,312,976; all patents enumerated herein incorporated by reference.
Technical Report AFML-TR-73-22 (Air Force Material Laboratory, Wright-Patterson AFB, Ohio) describes the production of polybenzimidazole from tetraaminobiphenyl and diphenyl isophthalate in the presence of various catalysts such as ammonium chloride, hydrochloride acid, p-toluenesulfonic acid, phosphoric acid, triphenylphosphate and boron trifluorideetherate.
In the Technical Report (pages 26-27) the data indicate that phosphorus-containing catalysts such as phosphoric acid and triphenyl phosphate are effective for increasing the inherent viscosity of a polybenzimidazole resin. However, there is formation of gel and insoluble black specks which tend to effect adversely the Plugging Value property of the polybenzimidazole products.
There remains a need for an improved method of producing polybenzimidazole which overcomes the various disadvantages of the prior art procedures, and which method yields an improved type of polybenzimidazole product particularly suitable for the formation of fibers having a high melting point and a high degree of thermal stability.
Accordingly, it is an object of the present invention to provide an improved two stage polymerization process for the production of high molecular weight polybenzimidazole in the presence of a phosphorus-containing catalyst.
It is another object of this invention to provide a high molecular weight polybenzimidazole which is characterized by an improved plugging value and an improved color specification.
Other objects and advantages of the present invention shall become apparent from the accompanying description and examples.